Our investigation is composed of two parts. Simultaneously we are studying the enzymology and the regulation of sterol biosynthesis. Enzymology - The microsomal enzymic steps in the conversion of lanosterol to cholesterol have not been established. Initially (a) we develop conditions for the study of an individual enzyme-catalyzed reaction in the process, (b) we study the nature of the reaction, (c) we isolate the enzyme from microsomes, and (d) we reconstitute segments of the biosynthetic pathway by appropriate recombination of purified enzymes with artificial membranes. With this approach to date in the study we have elucidated approximately one-half of the supposed 20 steps in the process. During future years we plan to complete the purification and reconstitution studies with full assembly of the enzymic system needed to catalyze the formation of cholesterol from lanosterol. Regulation - Recently we have developed evidence for a regulation in concert of the microsomal enzymes which includes HMG-CoA reductase as well as the enzymes that catalyze the terminal 22 reactions. A noncatalytic protein that participates in regulation has been purified to electrophoretic homogeneity. In the presence of the protein, end product inhibition can be observed. Work in progress should allow a complete description of the nature and function of the regulatory protein.